In electrostatography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image) is formed on an insulative surface by any of various methods. For example, the electrostatic latent image may be formed electrophotographically (i.e., by imagewise radiation-induced discharge of a uniform potential previously formed on a surface of an electrophotographic element comprising at least a photoconductive layer and an electrically conductive substrate), or it may be formed by dielectric recording (i.e., by direct electrical formation of a pattern of electrostatic potential on a surface of a dielectric material). Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
One well-known type of electrographic developer comprises a dry mixture of toner particles and carrier particles. Developers of this type are commonly employed in well-known electrographic development processes such as cascade development and magnetic brush development. The particles in such developers are formulated such that the toner particles and carrier particles occupy different positions in the triboelectric continuum, so that when they contact each other during mixing to form the developer, they become triboelectrically charged, with the toner particles acquiring a charge of one polarity and the carrier particles acquiring a charge of the opposite polarity. These opposite charges attract each other such that the toner particles cling to the surfaces of the carrier particles. When the developer is brought into contact with the electrostatic latent image, the electrostatic forces of the latent image (sometimes in combination with an additional applied field) attract the toner particles, and the toner particles are pulled away from the carrier particles and become electrostatically attached imagewise to the latent image-bearing surface. The resultant toner image can then be fixed in place on the surface by application of heat or other known methods (depending upon the nature of the surface and of the toner image) or can be transferred to another surface, to which it then can be similarly fixed.
A number of requirements are implicit in such development schemes. Namely, the electrostatic attraction between the toner and carrier particles must be strong enough to keep the toner particles held to the surfaces of the carrier particles while the developer is being transported to and brought into contact with the latent image, but when that contact occurs, the electrostatic attraction between the toner particles and the latent image must be even stronger, so that the toner particles are thereby pulled away from the carrier particles and deposited in the desired amount on the latent image-bearing surface. In order to meet these requirements for proper development, the level of electrostatic charge on the toner and carrier particles should be maintained within an adequate range.
Toner particles in dry developers often contain material referred to as a charge agent or charge-control agent, which helps to establish and maintain toner charge within an acceptable range. Many types of charge-control agents have been used and are described in the published patent literature. However, the level of charge that will be created and maintained on the toner is still very dependent on the nature and condition of the carrier particles.
Many known dry, two-component electrostatographic developers contain thermoplastic toner particles and carrier particles that comprise a core material coated with a fluorine-containing polymer, such as poly(vinylidene fluoride) or poly(vinylidene fluoride-co-tetrafluoroethylene). See, for example, U.S. Pat. Nos. 4,614,700; 4,546,060; 4,478,925; 4,076,857; and 3,970,571.
Such fluoropolymer carrier coatings can serve a number of known purposes. One such purpose can be to aid the developer to meet the electrostatic force requirements mentioned above by shifting the carrier particles to a position in the triboelectric series different from that of the uncoated carrier core material, in order to adjust the degree of triboelectric charging of both the carrier and toner particles. Another purpose can be to reduce the frictional characteristics of the carrier particles in order to improve developer flow properties. Still another purpose can be to reduce the surface hardness of the carrier particles so that they are less likely to break apart during use and less likely to abrade surfaces (e.g., photoconductive element surfaces) that they contact during use. Yet another purpose can be to reduce the tendency of toner material or other developer additives to become undesirably permanently adhered to carrier surfaces during developer use (often referred to as scumming). A further purpose can be to alter the electrical resistance of the carrier particles.
However, while such carrier coatings can serve all of the above-noted purposes well, in some cases they do not adequately serve some or all of those purposes simultaneously. For example, in some developer compositions, fluoropolymer carrier coatings can serve many of the above-noted purposes well, but, depending upon the nature of the toner particles and carrier core material desired to be included in the developer, such carrier coatings can cause the developer to acquire a triboelectric charge that is too high for optimum developer performance; i.e., the electrostatic latent image has difficulty pulling the toner particles away from the carrier particles. This is especially true in some positively charged developers (developers in which the toner particles triboelectrically acquire a positive charge, and the coated carrier particles acquire a negative charge).
Some prior patent publications describe means for alleviating this problem to some degree by blending the fluoropolymer with another modifying polymer having triboelectric characteristics different from the fluoropolymer and coating the blend on carrier core particles in order to further alter the carrier particles' triboelectric charging characteristics and, in some cases, provide other desirable properties, such as better adhesion of the coating to the core particles. Many different types of polymers have been described as useful for this purpose, among which are, for example, various styrene and methacrylate polymers and copolymers thereof. For example, U.S. Pat. Nos. 4,209,550; 4,297,427; and 4,590,140, suggest that, among many other polymers, poly(styrene), poly(methyl methacrylate), and poly(styrene-co-methyl methacrylate) may serve this purpose.
However, we have found that most of such polymeric materials exhibit one or more drawbacks when it is attempted to blend them with fluoropolymers and coat the blend on carrier core particles for this purpose.
For example, some of the suggested polymeric materials are not triboelectrically potent enough or different enough from the fluoropolymers to achieve the desired alteration in charging tendency of the carrier particles in certain developers. Also, the less triboelectrically efficient or potent the additional modifying polymer is for this purpose, the less of the fluoropolymer can remain in the blend in order to exhibit the desirable characteristics of fluoropolymer coatings noted above. For example, in the case where carrier core particles comprise stontium ferrite materials and have average particle diameters in the range of about 30 to 40 micrometers, it is desirable to be able to retain as much of the fluoropolymer in the coating as possible, and preferably at least about 1 part (by weight) of the fluoropolymer per 100 parts of carrier core material. However, one of the most desirable means of forming the coating on the core particles (often referred to as melt-coating) is to mix the core particles with finer particles of the coating material in solid form to distribute the coating particles over the core particles' surfaces, apply heat to cause the material to flow just enough to coat the core surfaces, allow the mix to cool, and then break apart the solidified mass to yield the discrete coated carrier particles. If the concentration of coating blend exceeds 3 parts per hundred parts (pph) of core material in the specific case noted above, the solidified mass becomes exceedingly difficult to properly break apart. Thus, since it is desirable in that case to include at least 1 pph of the fluoropolymer and undesirable to include more than 3 pph of total coating blend, the amount of modifying polymer that can be added is limited (it should be noted that the specific preferable minimum and maximum concentrations of coating material recited above will be different for different core particles that may have different average particle sizes, different core material densities, and/or different surface area-to-mass ratios). The more efficient the modifying polymer is at desirably altering the carrier particles' charging characteristics, the more desirable it is, in terms of achieving the desired charging characteristics and maximizing the amount of fluoropolymer within the practical upper limits of total blended coating material.
Another drawback of some materials that might be obvious to try as modifying polymers in the blend is their lack of thermal stability, leading to degradation during melt-blending at temperatures needed to properly coat the fluoropolymer (e.g., about 210.degree.-230.degree. C. in the case of poly(vinylidene fluoride)) and degradation during use in electrographic development, with consequent inconsistent triboelectric properties initially and over time and shorter carrier life (because of more carrier chipping, flaking, dusting, and scumming).
A further drawback of some possible modifying polymers is that the temperature range in which they will flow just enough to properly coat the carrier cores in a melt-coating process does not match or overlap the proper temperature range for the desired fluoropolymer, with possible consequences such as incomplete or non-uniform coating, poor coating adhesion, inconsistent carrier performance, and shorter carrier life.
Yet another drawback of some possible modifying polymers is the unexplained tendency of carrier particles coated therewith to cause unacceptably high levels of dusting in electrographic development use. Dusting (also referred to as throw-off) is defined as the amount of toner and any other particulate matter that is thrown out of the developer (i.e., that is not adequately held to the surfaces of the carrier particles) during agitation of the developer, e.g., by a typical development apparatus such as a magnetic roll applicator. High levels of dusting can involve undesirable effects such as excessive wear and damage of electrostatographic imaging apparatus, contamination of toner with dirt or carrier material leading to higher charge variation, contamination of environmental air with toner powder and other particulate matter, unwanted development of background image areas, and scumming of the surface of photoconductive elements that leads to poorer electrophotographic performance and shorter useful life.
Thus, there remains a need for suitable modifying polymers to be blended with fluorine-containing polymers and coated on carrier core particles to adjust their triboelectric charging characteristics with respect to various types of toner particles in electrographic developers. Such modifying polymers should be highly potent or efficient when blended with appropriate fluoropolymers in relatively small amounts in order to adequately modify carrier charging characteristics while retaining desirable properties imparted by the fluoropolymers, should have good thermal stability, should have proper flow characteristics for melt-coating in a temperature range matching or overlapping the proper coating temperature range for the fluoropolymers with which it is desired to blend them, and should not cause carrier particles to exhibit high dusting characteristics in electrographic developers. The present invention meets that need.